The invention relates to a new industrial process for producing low-monomer-content organic polyisocyanates by oligomerization of organic diisocyanates in a two-phase system.
The preparation of oligomeric polyisocyanates is known in the art (H. J. Laas et al, J. Prakt. Chem. 336, 185-200 (1994); EP 755 954, EP 798 299, EP 508 313). Hence a very wide variety of oligomerization reactions have been described in which some of the isocyanate groups present are consumed by reaction until a predetermined conversion rate is reached, after which residually present monomers are removed, for example, by distillation. As oligomerization reactions, a very wide variety of mechanisms of formation have become established, in the form of catalytic trimerization, dimerization, carbodiimidization, allophanatization, of biuretization, urea formation and urethanization of organic diisocyanates. Typically, in bulk, without solvent or diluent, 10% to 40% of the isocyanate groups of the diisocyanate component are subjected to the oligomerization reaction. When the reaction has been halted by deactivation of the catalyst, this generally leaves a relatively large fraction of the starting isocyanate in the reaction mixture. Subsequent distillative separation, in thin-film evaporators, for example, leads ultimately to products having a residual monomer content of below 0.5% or sometimes below 0.1%.
A disadvantage affecting reactions without solvent is the high reaction potential which, in the case of the highly exothermic reactions (involving a sharp increase in the temperature of the reaction mixture), can lead to an uncontrolled reaction course. From a process engineering standpoint, this must be guarded against by means of complicated operational shut-off mechanisms. Furthermore, there is a distinct rise in the viscosity during the reaction, producing considerable process engineering problems. All apparatuses must be designed for a wide temperature range and viscosity range, and hence does not operate in the optimum range.
Where the reaction potential is lowered by addition of relatively large amounts of solvents, the solvent remains in the reaction mixture and must be removed, if possible, by distillation, which is costly and inconvenient.
Where such reactions are carried out continuously, in one or more stirred tanks, for example, instances of back mixing in cascade reactors mean that the reaction must be terminated at an earlier stage than in the case of a batch reaction in order to obtain the same characteristics in the resulting polyisocyanates. This loss of conversion (increased amount of unreacted starting diisocyanate) must be compensated by a higher distillation performance and considerable extra complexity and expense. More favourable here would be a batch reaction without supply of fresh diisocyanate during the oligomerization. However, this mode of reaction, and hence the batch size, is limited by the need for heat removal and by the cooling systems that are technically realizable.
In the case where isocyanate reactions are carried out in a tube reactor, which is likewise a possibility, the large surface area of the tubes results in severe deposition and fouling phenomena, which can lead to a need for premature cleaning of the plant. The unavoidable formation of deposits leads to poor temperature transition at the tube walls and hence to relatively sharp temperature inhomogeneities. This affects not only the quality (e.g. colour) but also the yield of the product.
It was an object of the present invention, therefore, to provide a universal process for producing low-monomer-content polyisocyanates that eliminates these disadvantages.